1. Field of the Invention
The present invention relates to a technology for forming halftone dots used in printing.
2. Description of the Related Art
In high grade printing process, an original multi-tone image is converted into halftone images for primary ink colors, and these halftone images are then used to produce a print image. Halftone dots are generally used in the halftoning process for converting the original image into halftone images.
Conventional halftone dots (so-called AM halftone dots) are regularly arranged at certain intervals in a matrix fashion. The image density is expressed in terms of the total area of the halftone dots per unit area. In other words, the regular arrangement of AM halftone dots is fixed at all times, and the number of black pixels that constitute one halftone dot increases as the level of the multi-tone image signal increases. In this description, ‘pixel’ refers to one unit of recording in an image recorder. For example, in an image recorder that exposes photosensitive film or a printing plate using laser beams, one light spot of a laser beam corresponds to one pixel.
The following problems have been identified with conventional regularly arranged AM halftone dots:
(1) Moiré is likely to occur in periodic images.
(2) Rosette moiré is likely to occur in color printing.
(3) Tone jump is likely to occur when the halftone dot area rate is 50% in the case of square halftone dots because all adjacent halftone dots join each other at the same time throughout the image.
Consequently, in recent years, a halftone method called FM screening has quite often been used. In FM screening, the image density is increased not by increasing the halftone dot size but by increasing the number of small dots, which are called FM dots. Using FM screening, the above-mentioned various problems occurring in connection with AM halftone dots are reduced.
However, FM screening also has the following problems:
(1) The reduction of dot size makes it difficult to obtain stable printing performance.
(2) The increase of the dot size, on the other hand, would cause the image to exhibit noticeable roughness.
(3) The dot size is likely to change during printing process (in other words, the dot gain characteristic is unstable) because a (circumference/area) ratio of one FM screening dot is larger than the conventional halftone dot.